1. Field of the Invention
This invention relates to implantable prostheses and in particular to an improved sintered coating for such prostheses.
2. Description of the Prior Art
For at least the past twenty years, implantable prostheses have been coated with metal particles so as to provide a surface into which bone or tissue can grow. Such coatings have been applied by plasma spray and/or sintering. See, for example, Hahn, U.S. Pat. No. 3,605,123 and Pillier, U.S. Pat. No. 4,206,516. As a general rule, plasma spray coatings have a smaller void space than sintered coatings and thus are somewhat less preferred.
At present, a number of sintered coatings are in commercial use. The particles employed in these coatings are normally selected by sieving. They thus have a single-mode distribution about a mean diameter value. Typically, the mean value has been in the range from about 0.006 inches (150 microns) to about 0.040 inches (1000 microns). The breadth of the distribution about the mean will depend upon the sieves used to select the particles.
Sintered coatings of this type have been used in both single layer and multi-layer constructions. Also, they have been applied to prostheses having both smooth and stepped outer surfaces. See, for example, Noiles, U.S. Pat. No. 4,790,852.
One of the problems with the existing sintered coatings has been that relatively low levels of surface friction exist at the interface between the coating and the bone. Most prostheses are implanted by press fitting into a prepared cavity in the patient's bone. A high level of friction at the coating-bone interface is thus desirable because it helps provide a more stable initial fixation of the prosthesis in the prepared cavity. An initial stable fixation of the prosthesis has been found to increase the likelihood of bone ingrowth.